1. Field of the Invention
The present invention relates to a four-wheel steering apparatus which is utilized as a steering apparatus for an automobile to make its turning radius smaller or to maintain its traveling stability by changing not only the direction of the front wheel, but also the direction of the rear wheel when the automobile changes its driving lane or turns.
2. Related Background Art
In order to turn with ease in a narrow road, the turning radius of an automobile should be small. Also, the stability of the automobile should be maintained in changing lanes at a high-speed driving. To these ends, there has been in use in recent years a four-wheel steering apparatus whereby not only the front wheels, but also the rear wheels are moved when the steering wheel is operated.
In a conventionally known four-wheel steering wheel apparatus, the structure is provided with a front wheel steering angle detecting sensor for detecting steering angles given to the front wheels, a vehicle speed sensor for detecting vehicle speeds, a rear wheel output shaft for giving steering angles to the rear wheels by displacing itself in the axial direction, a rear wheel steering angle detecting sensor for detecting the rear wheel steering angles on the basis of the displacement of the rear wheel output shaft, an electric motor for causing the aforesaid rear wheel output shaft to be displaced over the axial direction, and a controller for controlling the energizing of this motor in response to the signals from each of the aforesaid sensors.
When steering angles are given to the rear wheels, the controller determines the steering angles on the basis of the signals form the front wheel steering angle sensor and the vehicle speed sensor and at the same time, energizes the aforesaid electric motor to cause the rear wheel output shaft to be displaced over the axial direction, thus giving the required steering angles to the rear wheels.
The steering angles thus given to the rear wheels are detected by the aforesaid rear wheel steering angle sensor. Then, the signals indicating the rear wheel angles are transmitted to the aforesaid controller. Thus, the controller controls the energizing of the electric motor so that the aforesaid rear wheel output shaft is stopped as it is while maintaining the state where the steering angles given to the rear wheels have reached the desired value. In other words, the aforesaid controller utilizes the signals from the rear wheel steering angle sensor as feedback signals to perform the so-called closed loop control of the aforesaid electric motor.
However, in the case of the conventional four-wheel steering apparatus which is structured to function as above, a plurality of the rear wheel steering angle sensors are needed including those to be used for fail-safe in order to allow the controller to perform the closed loop control of the electric motor. Accordingly, the structure becomes more complicated, hence unavoidably increasing the manufacturing cost of the apparatus.
In Japanese Patent Application Laid-Open No. 57-99470, Japanese Patent Application Laid-Open No. 58-214469, and Japanese Patent Application Laid-Open No. 58-214470, there are disclosed techniques to perform the so-called open control of the rear wheel steering angles in a four-wheel steering apparatus, in which steering angles are given to the rear wheels by a hydraulic cylinder or a pneumatic cylinder, without utilizing the operation of providing the rear wheels with appropriate steering angles in relation to the steering angles given to the front wheels as any feedback signals.
Nevertheless, in the case where a hydraulic cylinder or pneumatic cylinder is employed, a pump or compressor is required for providing hydraulic pressure or compressed air for steering the wheel and pipings are needed to supply the compressed air or hydraulic pressure, leading also to the unavoidable increase of the manufacturing cost of the apparatus.
When actually mounting in an automobile an electric four-wheel steering apparatus which provides the rear wheels with steering angles by the use of an electric motor such as disclosed in Japanese Patent Application No. 2-235553, it is preferable to enhance its safety in view of the points given below.
In the above-mentioned electric four-wheel steering apparatus, if a steering angle is given to the front wheels when the vehicle is driving at low speeds, a control is performed in order to make the turning radius of the vehicle smaller by providing the rear wheels with a steering angle in a direction opposite to the front wheels.
However, in a case where the vehicle speed sensor itself is damaged or the connecting wiring between the vehicle speed sensor and the controller is cut or some other malfunctions are caused to provide the controller with an information that the vehicle speed is zero (0 km/h) despite the vehicle is still in traveling, if a steering angle is given to the front wheels at that time, the rear wheels are still provided with a steering angle in the direction opposite to the front wheels even when the vehicle is traveling at high speeds.
Thus, there is a problem encountered in that the vehicle stability cannot be maintained sufficiently if steering angles are given to the rear wheels in the direction opposite to the front wheels when the vehicle is traveling at high speeds.
Also, in the above-mentioned electric four-wheel steering apparatus, a steering angle is given to the rear wheels with respect to a steering angle to the front wheels irrespective of the capacity of a battery when the steering angle is given to the front wheels.
Because the electric motor for providing the rear wheels with steering angles consumes a fairly large amount of electricity, if steering angles are given to the rear wheels while the battery capacity is not good enough, the capacity of the aforesaid battery becomes increasingly less, and there is a possibility that the vehicle driving can be hindered due to the inability of actuating the self-starting motor or the like when the vehicle is restarted after a stop.
Also, in a structure where no centering springs are provided to give bias to the rear wheels to return to the neutral position, if the battery is completely consumed in a state that a steering angle is still applied to the rear wheels, there is a possibility that the rear wheels remain with a steering angle being applied.